Structural characterization of fault damage zones in carbonates (Central Apennines, Italy)

The Italian Central Apennines are one of the most seismically active areas in the Mediterranean, hit in the last few centuries by several destructive earthquakes (i.e. L’Aquila 2009, Mw 6.3 e L’Aquila 1703, Mw 6.7; Avezzano 1915, Mw 7.1; Maiella 1706, Mw 6.8). The mainshocks and the aftershocks of these earthquake sequences propagate and often nucleate in normal fault zones NNW-SSE oriented, cutting km-thick limestones and dolostones formations Jurassic - Cretaceous in age. These normal faults, expression of the extensional tectonics that developed since late Pliocene on the previous compressive structures, present at their footwall few meters to hundreds of meters thick damage zones, characterized by a dense network of fractures and secondary faults. Despite their peculiarity and their geological interest: 1. the mechanism of formation of these damage zones, 2. their role in the seismic cycle, 3. their relationship during nucleation and propagation of individual seismic ruptures, 4. factors controlling their thickness, are still unknown. From these open questions and due to the absence of detailed studies about fault damage zones in carbonates, the necessity to investigate their structure arose. Previous high-resolution field structural surveys were performed in the last few years on the Vado di Corno fault zone (Campo Imperatore, L’Aquila) and Monte Marine fault zone (Aterno Valley, L’Aquila). Here we describe two minor carbonate-hosted faults located in the same area (Central Apennines): the Subequana Valley fault (Middle Aterno Valley, L’Aquila) and the Capo di Serre fault (Gran Sasso Massif, L’Aquila), both with the typical Apennine trend, and length (8-10 km) and displacement (200-300 m) smaller than the previous investigated cases (14-20 km in length, 1700-2000 m in displacement). For the structural characterization of these two faults, I performed two weeks of fieldwork to map the fault zones at scale 1:1000, collecting: - data relative to the main fault plane; - data relative to secondary faults in the damage zone; - joint sets in different structural stations; - samples for the description of the structural domains. I defined a fault zone rocks legend based on field observations and calibrated on the previous works (i.e. Vado di Corno and Monte Marine fault zone). From the collected data of each fault, a structural map and related geological cross-sections were realized. The description of the structural domains in the field was supported by the observations of polished hand-samples and thin sections at the optical microscope. Based on these preliminary observations, faults with similar length and displacement have different damage zones (i.e., thickness, presence/absence of a high strain damage zone): the Subequana Valley damage zone is at least 230 m thick in the point of maximum thickness along strike and has extended high strain and a low strain damage zones; the Capo di Serre damage zone has a maximum thickness of 120 m, with a prevalent low strain damage zone. The most relevant observations regard the damage zone thickness variations along strike; these variations, in terms of intensity and thickness of damage, are related to the presence of secondary structures and stepover zones between different segments of the main fault. As a consequence, the main factor controlling the thickness of damage zones is the presence of geometrical complexities such as inherited or secondary structures, rather than the depth of exhumation, the hosting lithology, and the accumulated displacement of the fault. Possible other factors having a key role in the development of these volumes of intensely deformed rock can be: - barrier effect on the rupture propagation in correspondence of the step-over zone; - interference between secondary structures and seismic waves. Data about damage zone thickness, length, and displacement of the two faults are finally compared to the previous investigated faults in the Apennines and to the other cases in the literature, with the aim to define possible scaling-relations.